Medium density polyethylene (MDPE) and high density polyethylene (HDPE) are widely used in polyolefin-based geomembrane applications. Those polyolefin compositions are relatively low cost. They have good physical property balances, chemical resistance, and ultraviolet (UV) weathering performance. They are easily processed in blown film and flat die extrusion processes.
However, an improvement in certain physical properties is often desired. Such properties include flexibility, impact resistance, dimensional stability, environmental stress cracking resistance (ESCR), and temperature resistance. Notably, geomembranes made with MDPE or HDPE tend to lack flexibility and impact resistance, especially in cold climate or cold environment applications.
Highly flexible elastoplastic polymer compositions provide an effective alternative to MDPE and HDPE for polyolefin-based geomembrane applications. Highly flexible elastoplastic polymer compositions provide a unique combination of flexibility, mechanical toughness, dimensional stability, barrier properties, and ESCR.
Highly flexible elastoplastic polymer compositions useful in geomembranes are taught in U.S. Pat. No. 5,286,564, U.S. Pat. No. 7,592,393, and United States Patent Application Publication No. US 2010/0305276. They have high flexibility, good chemical inertia, and good mechanical properties.
U.S. Pat. No. 5,286,564 describes highly flexible elastoplastic polymer compositions comprising, in parts by weight: A) 10-50 parts of an isotactic propylene homopolymer or copolymer, B) 5-20 parts of an ethylene copolymer fraction, insoluble in xylene at room temperature; and C) 40-80 parts of an ethylene copolymer fraction containing less than 40% by weight of ethylene, the fraction being also soluble in xylene at room temperature, and having an intrinsic viscosity from 1.5 to 4 dl/g; with a percentage by weight of the sum of the (B) and (C) fractions with respect to the total polyolefin composition from 50% to 90%, and a (B)/(C) weight ratio lower than 0.4. The compositions show a flexural modulus of less than 150 MPa and low values of Shore hardness.
U.S. Pat. No. 7,592,393 describes highly flexible elastoplastic polymer compositions comprising the following components, all percent amounts being by weight: A) 8-25% of a propylene homopolymer or copolymer; B) 75-92% of an elastomeric fraction comprising a first and a second elastomeric copolymer of propylene with other alpha-olefin(s), in a weight ratio from 1:5 to 5:1. The copolymers contain 45% or less of such alpha-olefin(s) and having specified values of solubility in xylene at room temperature and of intrinsic viscosity of the xylene-soluble fraction.
Such compositions show lower values of flexural modulus than the compositions disclosed in U.S. Pat. No. 5,286,564.
United States Patent Application Publication No. 2010/0305276 teaches a highly flexible elastoplastic polymer composition having an unusual and favorable balance of flexibility (low flexural modulus), softness and tensile properties, which is obtained when a specific copolymer of propylene with hexene-1 is combined with an ethylene copolymer having a high solubility in xylene at room temperature.
The polyolefin composition comprises the following components, all percent amounts being by weight: A) 15-40%, preferably 21-35%, more preferably 25-35%, in particular 26-35%, of a copolymer of propylene with hexene-1 containing from 2.5 to 10%, preferably from 6 to 10% of hexene-1 and having Melt Flow Rate (MFR, measured according to ISO 1133 at 230 degrees Celsius/2.16 kg) from 0.5 to 100 g/10 min. preferably from 0.5 to 70 g/10 min., more preferably from 2 to 70 g/10 min., in particular from 3 to 70 g/10 min.; B) 60-85%, preferably 65-79%, more preferably 65-75%, in particular 65-74%, of a copolymer of ethylene with propylene or a CH2=CHR alpha-olefin, where R is a C2-C8 alkyl radical, and optionally minor quantities of a diene, or a copolymer of ethylene with propylene and the alpha-olefin, and optionally minor quantities of a diene, the copolymers containing ethylene in a quantity equal to or lower than 40%, preferably from 15 to 40%, more preferably from 15 to 37%, and having solubility in xylene at room temperature greater than 70%, preferably greater than 80%, more preferably greater than 85%.
The amounts of (A) and (B) are referred to the total weight of the polymeric portion of the polyolefin composition.
It is desirable for MDPE- and HDPE-based compositions to be useful in preparing geomembranes and achieve physical property improvements similar to those of fPP-based geomembranes.